PHILADELPHIA — Sen. Hillary Rodham Clinton said emphatically Wednesday night that Sen. Barack Obama can win the White House this fall, undercutting her efforts to deny him the nomination by suggesting he would lead the party to defeat.

"Yes, yes, yes," she said when pressed about Obama's electability during a campaign debate six days before the Pennsylvania primary.

Asked a similar question about Clinton, Obama said, "Absolutely, and I've said so before" — a not-so-subtle response to suggestions from his rival that he could not defeat Republican Sen. John McCain.

Both rivals pledged not to raise taxes on individuals making less than $200,000 and said they would respond forcefully if Iran obtained nuclear weapons and used them against Israel.

"An attack on Israel would incur massive retaliation by the United States," Clinton said.

Obama said, "The U.S. would take appropriate action."

A gap on Social Security
They differed over Social Security when Obama said he favored raising payroll taxes on higher-income individuals. Clinton said she was opposed. Her rival quickly cut in and countered that she had said earlier in the campaign she was open to the idea.

Under current law, workers must pay the payroll tax on their first $102,000 in wages. Obama generally has expressed support for a plan to reimpose the tax beginning at a level of $200,000 or more.

The debate was the 21st of the campaign for the nomination, an epic struggle that could last weeks or even months longer.

Pennsylvania, with 158 delegates at stake, is a must-win contest for Clinton, who leads in the polls and hopes for a strong victory to propel her through the other states that vote before the primary season ends June 3.

Video: Todd: ‘Tough night for Obama’
Obama leads Clinton in the delegate chase, 1,647-1,511, according to msnbc.com and NBC News calculations. Earlier in the day, he picked up the endorsements of three superdelegates from two states with primaries on May 6 — Reps. Andre Carson of Indiana and Mel Watt and David Price of North Carolina.

After primaries and caucuses in 42 of the 50 states, Obama leads his rival in convention delegates, popular votes and states won. She is struggling to stop his drive to the nomination by appealing to party leaders who will attend the convention as superdelegates that he would preside over an electoral defeat at a moment of great opportunity after eight years of Republican rule.

'It's going to be Barack or me'
The former first lady has never denied published reports that she once told New Mexico Gov. Bill Richardson that Obama couldn't win when he called to tell her he would be endorsing the Illinois senator.

And at a news conference earlier this month in California, Clinton sidestepped when asked directly whether Obama would win if he were the Democratic nominee. "I am sure we will have a united Democratic Party. I will do everything possible to make sure we can win and I am confident we will have a Democrat in the White House next year," she said at the time.

Asked a similar question at the debate, she provided a similar answer at first. "I think we have to beat John McCain, and I have every reason to believe we're going to have a Democratic president and it's going to be Barack or me."

Pressed by George Stephanopoulos of ABC News to answer the question directly, she said, "Yes, yes, yes.... Now, I think I can do a better job."

Apology for Bosnia gaffe
In a debate that moved swiftly between politics and policy, Clinton issued a first-ever public apology for having claimed erroneously that she landed under sniper fire in Bosnia in 1996 as first lady.

"I may be a lot of things, but I am not dumb," she said, adding that she had written in her book that there had been no gunfire during the episode. She said she was embarrassed by her error. "I'm sorry I said it," she added.

She previously had explained her incorrect comments by saying she had misspoken.

Obama later erred by saying he had never favored a ban on handguns even though as a state Senate candidate in 1996 he filled out a questionnaire from an Illinois voter group saying he would support such a ban.

"My writing wasn't on that particular questionnaire ... as I said, I have never favored an all-out ban on handguns," Obama said, even though his handwritten notes did appear on its front page. The reponse to the question about guns was typed.

Explaining the 'bitter' comment
It was his turn to explain a few moments later, when asked about his controversial comment that small-town Americans become bitter because of economic adversity and "cling to guns or religion or antipathy to people who aren't like them" as a result.

He said he was attempting to say that because voters feel ignored by government, "they end up being much more concerned about votes around things like guns where traditions have been passed on from generation to generation. And those are incredibly important to them."

"People don't cling to their traditions on hunting and guns" out of frustration with their government, Clinton said. She added that Obama had a fundamental misunderstanding on the role of religion and faith.

"I think very highly of Senator Clinton's record, but I think it is premature at this point to talk about who the vice presidential candidates will be because we're still trying to determine who the nominee will be," Obama said.

Clinton was similarly noncommittal. "I'm going to do everything I possibly can to make sure that one of us takes the oath of office next January. I think that has to be the overriding goal," she said.

Neither was willing to say President Bush would be asked to serve in any capacity after he leaves office. Obama volunteered he would be "more likely to ask the advice of the current president's father. He said, that as president, George H. W. Bush had presided over a "wise foreign policy" at the time the Cold War was ending.

ABC News sponsored and televised the debate, with Charles Gibson and Stephanopoulos moderating.

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